1. Field of the Invention
The present invention relates to a valve operating device for an internal combustion engine, which is capable of changing the operating characteristics of engine valves.
2. Description of the Prior Art
A valve operating device for an internal combustion engine has already been known, for example, from Japanese Patent Application Laid-open No. 100210/88, which includes a plurality of rocker arms disposed adjacent one another for swinging movement about a common axis, a plurality of cams provided on a cam shaft in independent correspondence to the rocker arms, and a connection switchover means capable of switching over the connection and disconnection of a combination of the rocker arms.
In the connection switchover means of such valve operating device, a hydraulic pressure is applied to a hydraulic pressure chamber from one axial direction of switchover members slidably fitted in the rocker arms and connected to one another, and the spring characteristic of a return spring acting in the other axial direction of the switchover members is changed at a plurality of stages in order to enable the sliding stroke of each switchover member to be switched over at a plurality of stages by switching over the hydraulic pressure applied to the hydraulic pressure chamber at a plurality of stages. However, in order to enable the connection and disconnection of the adjacent rocker arms to be switched over at each of the sliding strokes of the switchover members, each of the switchover members must be formed into a stepped configuration, resulting in a troublesome machining. Moreover, in the prior art device, the rocker arms are not in their connected states in a condition in which each of the switchover members has not been slid by hydraulic pressure. Therefore, if a free rocker arm capable of being freed relative to the engine valves is disposed between a pair of driving rocker arms operatively connected to the engine valves and corresponding to cams for substantially stopping the engine valves, when the connection switchover means has been brought into its inoperative state due to any cause in an operating range in which the engine valves should be driven by the free rocker arm, the free rocker arm cannot be connected to any of the driving rocker arms, and when the cams corresponding to the driving rocker arms are arranged to substantially stop the engine valves, the engine valves are also brought into their substantially stopped states.
In the above prior art device, all the switchover means are simultaneously operated in a switching manner and hence, the degree of freedom of the connection and disconnection of the rocker arms in combination is limited. In order to change the various operating characteristics of the engine valves, it is desirable to increase the degree of freedom of the connection and disconnection of the rocker arms in combination.
A valve operating device for an internal combustion engine has already been also known, for example, from Japanese Patent Publication No. 75729/91, which includes a driving rocker arm operatively connected to an engine valve, first and second free rocker arms adjacently disposed on opposite sides of the driving rocker arm, so that they can be freed relative to the engine valve, first and second cams provided on cam shaft in independent correspondence to the free rocker arms and having cam profiles intersecting each other, and connection switchover means capable of switching over the connection and disconnection of the driving rocker arm to and from the free rocker arms.
In this device, the engine valve is opened and closed relatively slowly in a high-speed operating range of the engine to insure a sufficient opening area desired by the engine, and the engine valve is opened and closed relatively rapidly in a low-speed operating range of the engine to insure a sufficient opening area desired by the engine, by switching over a state in which the first free rocker arm is connected to the driving rocker arm operatively connected to the engine valve to open and close the engine valve by the first cam and a state in which the second free rocker arm is connected to the driving rocker arm operatively connected to the engine valve to open and close the engine valve by the second cam. However, in switching over the state in which the driving rocker arm is connected to the first free rocker arm and the state in which the driving rocker arm is connected to the second free rocker arm, the switching operation should be completed at one timing when the first and second rocker arms have been stopped by base circle portions of the first and second cams. However, when both the connection switchover means have been brought into their connecting states at a displaced timing of switching, an abnormal behavior such as a valve jumping may be produced in the engine valve due to the intersection of the profiles of the first and second cams for swinging the first and second free rocker arms.
Further, a valve operating device for an internal combustion engine has already been known, for example, from Japanese Patent Publication No. 38887/92, which includes a rocker arm swingably carried on a rocker arm shaft and having a support sleeve integrally provided thereon with its inner surface put into sliding contact with an outer surface of the rocker arm shaft, another rocker arm swingably carried on the support sleeve, an engine valve operatively connected to at least one of the rocker arms, and a connection switchover means provided between the support sleeve and the other rocker arm and capable of switching the connection and disconnection of the rocker arms from one to another in response to the switching operation of a switchover piston having an axis perpendicular to an axis of the rocker arm shaft.
In such valve operating device, the switchover piston having the axis perpendicular to the axis of the rocker arm shaft is fitted into the support sleeve for sliding movement between a connecting position in which it is in with the rocker engagement arm carried on the support sleeve and the engagement with the rocker arm is released. For this reason, the support sleeve must be increased in size and correspondingly, the rocker arm swingably carried on the support sleeve is also increased in size, resulting in an increased inertial moment. When the rocker arm is being swung in the disconnecting state, a centrifugal force is applied to the switchover piston outwardly in a radial direction of the rocker arm shaft and hence, when the spring force of a return spring for biasing the switchover piston toward a disconnecting position is small, a tip end of the switchover piston is urged against an inner surface of the rocker arm by such centrifugal force, resulting in an increased wear between the switchover piston and the support sleeve. If the spring force of the return spring is increased, the hydraulic pressure force applied to the switchover piston during connection must be increased. In a high-speed rotational range, it is difficult to overcome the wear problem even by the increase in spring force of the return spring.
A connection switchover means having an operating axis perpendicular to an axis of the rocker arm shaft is disclosed in Japanese Patent Application Laid-open No. 72403/92. In this connection switchover means, a pair of rocker arms are adjacently disposed on opposite sides of a rocker arm integral with a rocker arm shaft to abut against cams having different profiles, and connection switchover means provided between the rocker arm shaft and the rocker arms disposed on the opposite sides, respectively. In this connection switchover means, a problem of an increase in size of the rocker arms and a problem of a wear are not arisen, but a combination of the rocker arm integral with the rocker arm shaft and the rocker arms disposed on the opposite sides of such rocker arm is disposed for every cylinder and hence, in a multi-cylinder internal combustion engine, hydraulic pressure circuits leading to oil passage provided in the rocker arm shafts in cylinders must be provided in a cylinder head, resulting in a complicated arrangement of the hydraulic pressure circuits in the cylinder head.